Wide band amplifier



INPUT 4 16 WA i E 2 Nov. 2 194. s. clszlKLAi WfDE BAND AMPLIFIER FiledDeg. 29, 1944 34' S/GIVAL 11 Tuenssr FREQUENCY r0 BE AMPZ/HED IN VENTOR. 650/9 6' JZl/(AA/ ATTOENEV Patented Nov. 2, 1948 UNITED STAT ESPATENT OFFICE wine BAND AMPLIFIER George C. Sziklai, Princeton, N. 3.,assignor to Radio Corporation of of Delaware America, a corporationApplication December 29, 1944, Serial 6. 570,295

The present invention relates to amplifying systems, and moreparticularly to an improved wide band amplifier coupling networksuitable for use as a video amplifier in televisionsystems and circuits.

In television systems and circuits it is necessary to amplify signalfrequencies occupying a Wide range of the frequency spectrum from thevery low frequencies up to frequencies of three or four megacycles. thatthe gain of an amplifier stage may be made more uniform over a givenfrequency range by employing filter circuits as an inter-sta e couplingmeans. It is also well knownthat the product of the band width and theamplification 1 ratio is dependent primarily on the transductance of theamplifier tube and of the capacities to ground. The product of the bandwidth and=the amplification ratio is limited bythe ratio of thetransductance Gm/c, in which 0 is the shunt capacitance. The efiect ofthe shunt capacitance decreases as the frequency increases. For example,the reactance of the shuntcapacitance, whether it is the inherentcapacitance of the tube or a separate physical condenser, decreases atafairly uniform rate as theifrequency is increased. By using filtercircuits, one may proinvention is to provide a simple and economicalcircuit means for efficient high frequency filter compensation.

Another and related object of the invention is to provide novel circuitmeans and to render: the alignment of these circuit means easy ofaccomplishment.

It is by now well known :1

3 claims. (01. 179-171) 21. An adjustable condenser3 4xserves asabridgingxcapacitor for the filter and has a valueof ap- A furtherobject is to provide a filter circuit having a constant gain over thepass band, the gain being increased by adding the gain-band width arearemoved above the cut-off frequency to the area in the pass band. i

Other objects and advantages of the invention will, of course, becomeapparent and immediately suggest themselves to those skilled in the artI to which the invention is directed from a reading of the followingspecification in connection with the accompanying drawings in which:

Fig. 1 shows schematically a coupling circuit in accordance with theinvention; and

connected to an output terminal ll. IZ of the tube I0 is supplied withsignals from a terminal l4 whichmay be connected to the Fig. 2 showscurves serving toillustrate the advantageous features of the circuits ofFigpl.

Referring for the present to Fig. 1 ofithe drawings, N] indicates asignal source, such as a vacuum tube amplifier, repeater or the like,the out- I put of which is to 'be coupled to a succeeding stage ortranslating device (not shown) which maybe The grid output terminal of apreceding stage of amplification orto asource ofimage signals, such as acamera tube or the like. In the event thatthe grid [2 is connected to aprecedingstage of amplification, it willbe understood thata filter cou-.pling arrangement, such was the filter 2|", constituting the inventionandto bedescribed hereinafter, will be substituted for the simplearjrangement shown which comprisesfia coupling condenser l6 and a gridresistor l8. Coupling between the outputof the signal source Ill, which,

'in the illustrative embodiment, is the plate. circuit IQ of the vacuumtube I0, is effected by a l circuit organization comprising thepreviously mentioned band pass filter 2 I. The filter is made upofseries inductances 24 and26 andparallel capacitances 21, 28, and 3|.

The capacitances 21 and 28 are shown by dotted linesand represent theinherent input and output capacitances of the stages to be coupled. Inthe present instance, the capacitance 21 isitheanode-to-cathode'capacitance of the tube Ill, and ca'pacitanceiill isthe input capacitance of the translating device or succeeding stagewhich is in communication with the terminal I l. The filterZlisterminated by a resistor 33. l 3

The condenser 3! is a balancing capacitance having a value approximatelyequal to that of proximately The output connection! I is taken acrossthe midshunt arm which comprises the condenser 28.

The filter 2l' is formed from a constant-K type ladder filter with theaid of a bridging capacitor,

and thus possesses a distinct advantage over the m-derived ladder filterwhich requires at least one additional inductor and is more difficult toadjust. Since filters of the constant K type have not providedsuflicient gain over a sufiiciently large portion of the frequencyspectrum to permit their most efiicient use, the relatively more laddertype m-derived filter without attendant disadvantages; The curves ofFig. 2 are obtained by plotting frequency f against gain indicated'as Gon the ordinate scale. Curve 34 shows the gain obtained when a simple RCcircuit is employed for coupling. An example of such a circult would bethe plat load resistor shunted by the cathode-to-plate capacitance of avacuum tube. of the frequency range is useful since the gain falls offrapidly. Also, there is no Well defined cut off region, and therefore,the area under the curve 23, while it is large, does not represent auseful gain-band width area.

An ideal response which may not be in practice is indicated by the,rectangle 36. Here,

.the'c utofls point is abrupt and all of the gainbandwidth areaisavailable. The area under the curve 34 is approximately equal to thearea under the curve 36, it being remembered that curve 34 stillliesabove thezero axis at a remote, point, theoretically, at infinitefrequency, From these considerations it appears that to solve theproblem of increasing the available gain-band width area, itis'desirable toincreasethe gain obtained when a filter is employed byadding the gainband width area removed above the cut off frequency tothe area in the pass band.

:Curve 38 is the curve which would be obtained by-employing atwo-section constant K filter, for

example. This-may be of the type employing a resistor and a peaking coiland capacitances, such as the inherentinput and output capacitances oftheinput and output circuits and/or the inherent capacities-oftheinductive and resistive elements themselves. Very considerableimprovement-in gain at the higher frequencies is to be observed.However, this curve represents, but to a lesser degree, a non-usableportion of the. curve i n-a region where attenuation is high at higher:frequencies. Considering Fig. .1 of'the drawings,

afilter constitutingonly the elements 24, 26, 21,

'28, and 3! with the output taken across the terminating resistor 33,would give a characteristic somewhat as shown by the curve 38. Someimprovement would be obtained byconnecting the output connection acrossthe .mid -shunt arm as shown. e

"In accordance Withthe invention, the capacitor-'34; the value'ofwhichwith respect to other circuit elements is discussed above, isbridged across. the inductances 24 and 26, the result being acharacteristic indicated byreference character 39 Byadding-this-capacitor it will be observed that a considerable portion ofthe gain-band width area is'removed above the cut off frequency and ismade available in the pass hand. Moreover, the curve 39.more nearlyapproaches the ideal curve 36.

.The circuit just described is particularly useful when the capacitance28 is greater than twice the capacitance 21, without the addition ofcapacitors; which is a common case.

attained I off efiect not only is retained but takes a more It will benoted that a very small part desirable form for video amplifiercharacteristics. Various modifications of the invention shown i anddescribed herein by way of example are possible without departing fromthe spirit and scope of the described invention, and it is desired thatany and all such modifications be considered within the purview of thepresent invention defined by the hereinafter appended claims.

Having now described the invention, What is claimed and desired to besecured by Letters Patent is the following: i .1. A video amplifieremploying abridged-T network 'for' high frequency correction.comprising, a signal source serving as an input for said network, saidnetwork being of the low pass} type composed of inductors inseriesbridgedibycae pacitors, a terminal resistorior said networkgan outputconnection to said network at the location of an intermediate shuntarm,and an adjustable balancing capacitance bridged across said inductancesin series whereby to cause the out 01f of said network to bemore abruptso -that a greater portion ofthe gain-band width area of theresponsecurve of said network is avai-lablefor signal transfer. 1

2 A video amplifieremploying a bridged-T network for highfrequencycorrection, the network comprising a pair of seriesinductances, shunt capacitances in said'network effectivelyat theinputand output ends of said inductances and at theirmid-connection, anoutput connection provided across the capacitance at the mid-comnection, and a capacitance bridging said inductances.,

GEORGEC. szIKnAL REFERENCES orTE D The'following references are ofrecord iii-the file of this patent:

UNITED STATES PATENTS;

, Number Name 1 i'Date 1,896,468 Shoemaker Feb. '7, 1933 2,093,665Tellegen l Sept;21, 1937 2,269,693 Schade Jan. 13, 1942 2,269,694 Schadee i- Jan. 13, 1942 2,315,784 Goodale Apr. 6,.1943

